This is an epidemiologic study of 1458 human females exposed in uterto to maternal diagnostic x-ray (pelvimetry etc.), each with an unexposed control, matched by hospital of birth, parity, and birthdate. They were born 1947-1952 in Baltimore hospitals, and are now Baltimore residents. Our objective is to determine whether x-ray during fetal life leads to changes that could be observed in young adults during their reproductive years and in their offspring. Animal experiments show that fetal and infant germ cells pass through stages of great sensitivity to radiation damage. Depletion of the finite supply of oocytes, with accelerated development of some surviving cells, may result in a temporary increase in fertility, with subsequent sterility and ovarian changes. Increased loss or malformation of F2 offspring may occur. There is evidence from animal experiments and from Hiroshima- Nagasaki that in utero exposure may retard growth and mental development. Phase I of this study, by ascertainment of registered Baltimore births through 1970 to the study population, demonstrated a 10-15% increased fertility among exposed women. The difference remained after adjustment for economic, social, and medical factors. No significant difference in F2 survival was found. Phase II, now starting, is a direct follow-up of 479 matched pairs, to verify Phase I findings, and to study possible differences in growth, development, and health of exposed versus control individuals, and of their children. Results of Phase I have been published.